Preheat oven to 375 degrees F. In 13" by 9" glass baking dish, arrange one-third of lasagna noodles, overlapping to fit. Spread half of ricotta mixture over noodles. Drain off liquid from spinach if any; spoon half of the spinach over ricotta mixture; then top with one-third of the sauce. Repeat layering. Top with remaining noodles, then remaining sauce.